The present invention relates to novel polyimide and polyisoimide resins end-capped with diaryl substituted acetylene (DASA) having high fusion temperatures, and to their use in composites, molding compounds, electronics and adhesives.
Polyimides are synthetic organic resins characterized by repeating imide linkages in the polymer chain which may or may not be end-capped with polymerizable or inert (i.e. non-polymerizable) chemical groups. They are available in both linear and crosslinked forms and are noted for their outstanding chemical and physical properties, particularly their high temperature oxidative stability and strength. In addition to their use as adhesives and molded articles, they may be used as precured films and fibers, curable enamels, laminating resins, and as matrices for fiber reinforced composites.
For many years polymer chemists have sought to attain materials which have a combination of three properties: 1) high use temperature; 2) toughness; and 3) processibility. In many applications, if high use temperature is required, the polymer must have a high Tg (glass transition temperature) as well as sufficient thermo-oxidative stability below its Tg. Aromatic polyimides have some of the highest use temperatures. However, prior art polyimides generally do not possess all three desired properties. Condensation polyimides are well known, see J. P. Critchley, G. J. Knight and W. W. Wright, "Heat-Resistant Polymers," Plenum Press, New York (1983), p. 185. Thermoplastic polyimides are generally tough, but either can be used at high temperatures (above 250.degree. C.) or are easily processed, but not both. Thermoplastic polyimides with use temperatures above 250.degree. C. must generally be processed at temperatures at which decomposition can become a problem (above 400.degree. C.) in order to achieve an acceptably low melt viscosity. Even then, high melt viscosity can still create difficulties (see N. J. Johnston and T. L. St. Clair, SAMPE Journal, January/February 1987, p. 12).
To minimize problems with viscosity, lower molecular weight thermosetting polyimides have been used. The prior art discloses polyimide oligomers (n=3-11, where n is the number of repeat units in the polymer backbone) containing acetylene in the backbone, with uncured Tg's up to 196.degree. C. See Macromolecules, 19, 2108 (1986). A low polyimide oligomer (n=0) with a phenyl-substituted acetylene end-cap and an uncured Tg of 111.degree. C. is disclosed in ACS Polymer Preprints, 26 (1), 136 (1985). A polyimide oligomer (n=1) containing 5,5'-[2,2,2-trifluoro-1-(trifluoromethyl)ethylidene]bis-1,3-isobenzofurand ione (6FDA) [alternatively named 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride], 1,3-bis(3-aminophenoxy)benzene (APB), and end-capped with 3-phenylethynylaniline, and with an uncured Tg of 120.degree. C. is disclosed in ACS Polymer Preprints, 25 (1), 110 (1984), ACS Polymer Preprints, 24 (2), 324 (1983), J. Macromol. Sci .--Chem., A21 (8+9), 1117 (1984).
Polyimides with unsubstituted acetylene end-caps such as aminophenylacetylene are described in U.S. Pat. Nos. 4,506,100 and 4,644,040. U.S. Pat. Nos. 3,845,018 and 3,879,349 issued on Oct. 29, 1974 and Apr. 22, 1975, respectively to N. Bilow et al.; and U.S. Pat. No. 4,405,770 issued on Sept. 20, 1983 to J. Schoenberg also described the preparation of aminophenylacetylene capped polyimide oligomers. See also Hergenrother, P. M., "Acetylene-terminated Imide Oligomers and Polymers Therefrom", Polymer Preprints, Am. Chem. Soc., Vol. 21 (#1), pgs, 81-83 (March 1980). European Patent Application No. 71,372 issued on Feb. 9, 1983 to A. Landis describes the preparation of aminophenylacetylene capped polyisoimide oligomers.
Most of the prior art thermosets tend to be brittle. It is well known that less brittle materials are obtained when the density of crosslinks is reduced (or, alternatively, the molecular weight between crosslinks is increased). However, as the molecular weight of the thermoset increases, its Tg in the uncured state will rise, and its Tg in the cured state will fall. The latter can be offset by changing the backbone of the thermoset to a more rigid structure thus raising the cured Tg. However, this will further increase the uncured Tg. At a sufficiently high uncured Tg the material will lose its processibility, not due to a high viscosity, but due to the fact that the curing reaction becomes too rapid at the required processing temperature. Thus the available processing time becomes too short.
Hence, there exists a need for novel polyimides and polyisoimides terminated with diaryl substituted acetylene (DASA) end caps which have high cured Tg, and toughness (via longer oligomers), while maintaining their processibility.